A slicer is usually used in a digital data receiver to determine the logical value of an input data signal. For a slicer to make a correct decision, an input signal representing a logic 1 must be sufficiently apart from an input signal representing a logic 0. As such, the input data signal is required to meet a certain minimum input eye mask requirement so the slicer can determine whether an input signal is representing a logic 1 or a logic 0. Usually, the signal amplitude difference (or eye height) is used as such a minimum eye mask requirement and is designated as VEH. At the same time, some noise immunity is preferred in that when the input signal level is below a certain limit, it is considered noise and the slicer is not required to respond to it. There are several known schemes to achieve such noise immunity, one of which is to design a comparator with built-in hysteresis.
Refer to the example shown in FIG. 1, given a minimum eye opening requirement, one can design a slicer for maximum noise immunity for any input signal that meets the minimum eye opening requirement. Such a slicer has a rising edge threshold Vthr close to the top the minimum eye mask and a falling edge threshold Vthf close to the bottom of the minimum eye mask respectively. The noise immunity of the slicer is thus Vhyst=Vthr−Vthf. As depicted in FIG. 1, the slicer can have a maximum possible noise immunity of Vhyst,max=VEH.
FIG. 2 illustrates how a conventional slicer makes a decision as to whether the input signal is a logic 1 or logic 0 in an ideal case when there is no noise. The top waveform depicts the input data signal and the bottom the corresponding output of the slicer having rising edge threshold Vthr and falling edge threshold Vthf. As depicted, point 201 represents a point at the rising curve of the input signal if sampled the sampled value is equal to Vthr of the slicer. For any point on the rising curve of the input signal, if the sampled value is lower than that of point 201, the slicer outputs a logic 0. The slicer outputs a logic 1 only when the sampled value of the input signal equals to or is greater than Vthr. Point 202 represents a point on the falling curve of the input signal if sampled the sampled value is equal to Vthf of the slicer. As shown, the slicer outputs a logic 1 as long as the sampled input is greater than Vthf, Only when the sampled input falls below the Vthf does the slicer outputs a logic 0.
However, in complex systems, the corresponding input signals to the slicer not only tend to degrade as external noise increases but also shift in signal level due to, for example, unequal ground reference between the transmitter and receiver, Consequently, the resulting signal eye opening at the receiver shrinks, as shown in FIG. 3. If the signal is shifted up/down by Vos, the available eye opening for a conventional slicer hysteresis is reduced to Vhyst,conv=VEH−2*Vos, Therefore the noise immunity is reduced as a result of the shifting of Vos. Even worse, when Vos>VEH/2, the conventional slicer scheme ceases to function correctly since no decision level can be set such that the comparator can determine the input data correctly.
Accordingly there remains a need in the art for a solution that addresses the problems above among others.